534: 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



considerable importance, it will be well to sketch 

 briefly the theory I now hold, and afterward show 

 its application to some of the detailed cases ad- 

 duced in Mr. Darwin's work. 



The very frequent superiority of the male bird 

 or insect in brightness or intensity of color, even 

 when the general tints and coloration are the 

 same, now seems to me to be due to the greater 

 vigor and activity and the higher vitality of the 

 male. The colors of an animal usually fade dur- 

 ing disease or weakness, while robust health and 

 vigor add to their intensity. This intensity of 

 coloration is most manifest in the male during the 

 breeding-season, when the vitality is at a maxi- 

 mum. It is also very manifest in those cases in 

 which the male is smaller than the female, as in 

 the hawks and in most butterflies and moths. 

 The same phenomena occur, though in a less 

 marked degree, among mammalia. Whenever 

 there is a difference of color between the sexes 

 the male is the darker or more strongly marked, 

 and difference of intensity is most visible during 

 the breeding-season (" Descent of Man," p. 533). 

 Numerous cases among domestic animals also 

 prove that there is an inherent tendency in the 

 male to special developments of dermal append- 

 ages and color, quite independently of sexual or 

 any other form of selection. Thus, " the hump 

 on the male zebu cattle of India, the tail of fat- 

 tailed rams, the arched outline of the forehead 

 in the males of several breeds of sheep, and the 

 mane, the long hairs on the hind-legs, and the 

 dewlap of the male of the Berbura gcat," are all 

 adduced by Mr. Darwin as instances of charac- 

 ters peculiar to the male, yet not derived from 

 any parent ancestral form. Among domestic 

 pigeons the character of the different breeds is 

 often most strongly manifested in the male birds ; 

 the wattle of the carriers and the eye-wattles of 

 the barbs are largest in the males, and male 

 pouters distend their crops to a much greater 

 extent than do the females, and the cock fantails 

 often have a greater number of tail-feathers than 

 the females. There are also some varieties of 

 pigeons of which the males are striped or spotted 

 with black, while the females are never so spotted 

 ("Animals and Plants under Domestication," I., 

 161); yet in the parent stock of these pigeons 

 there are no differences between the sexes either 

 of plumage or color, and artificial selection has 

 not been applied to produce them. 



The greater intensity of coloration in the 

 male — which may be termed the normal sexual 

 difference — would be further developed by the 

 combats of the males for the possession of the 



females. The most vigorous and energetic usu- 

 ally being able to rear most offspring, intensity 

 of color, if dependent on or correlated with 

 vigor, would tend to increase. But as differences 

 of color depend upon minute chemical or struct- 

 ural differences in the organism, increasing vig- 

 or acting unequally on different portions of the 

 integument, and often producing at the same 

 time abnormal developments of bair, horns, scales, 

 feathers, etc., would almost necessarily lead also 

 to variable distribution of color, and thus to the 

 production of new tints and markings. These 

 acquired colors would, as Mr. Darwin has shown, 

 be transmitted to both sexes or to one only, ac- 

 cording as they first appeared at an early age, or 

 in adults of one sex, and thus we may account 

 for some of the most marked differences in this 

 respect. With the exception of butterflies, the 

 sexes are almost alike in the great majority of 

 insects. The same is the case in mammals 

 and reptiles, while the chief departure from the 

 rule occurs in birds, though even here in very 

 many, cases the law of sexual likeness prevails* 

 But in all cases where the increasing develop- 

 ment of color became disadvantageous to the 

 female, it would be checked by natural selection, 

 and thus produce those numerous instances of 

 protective coloring in the female only which oc- 

 cur in these two groups of animals. 



There is also, I believe, a very important 

 purpose and use of the varied colors of the high- 

 er animals, in the facility it affords for recognition 

 by the sexes or by the young of the same species ; 

 and it is this use which probably fixes and de- 

 termines the coloration in many cases. When 

 differences of size and form are very slight, color 

 affords the only means of recognition at a dis- 

 tance or while in motion, and such a distinctive 

 character must therefore be of especial value to 

 flying insects which are continually in motion, 

 and encounter each other, as it were, by accident. 

 This view offers us an explanation of the curious 

 fact that among bulterflies the females of closely- 

 allied species in the same locality sometimes 

 differ considerably, while the males are much 

 alike ; for as the males are the swiftest and the 

 highest fliers and seek the females, it would evi- 

 dently, be advantageous for them to be able to 

 recognize their true partners at some distance 

 off. This peculiarity occurs with many species 

 of Papilio, Diadema, Adolias, and Colias. In 

 birds such marked differences of color are not 

 required, owing to their higher organization and 

 more perfect senses, which render recognition 

 easy by means of a combination of very slight 



